Electromagnetic radiation absorbers and modulators comprising polyaniline

ABSTRACT

A method is provided for absorbing electromagnetic radiation using a polyaniline composition, wherein the radiation is infrared, visible light, ultraviolet, radar, or microwave radiation. Also provided is a method for reducing the detectability by radar of an object by increasing the absorption of radar waves and thereby decrease the reflection of the radar waves by applying to the object a polyaniline composition or a partially protonated salt thereof.

This invention was made with government support under ContractN00014-86K-0766 awarded by the Department of Navy. The government hascertain rights in the invention.

This is a division of application No. 07/764,236, filed Sept. 23, 1991,now U.S. Pat. No. 5,147,968, issued Sept. 15, 1991, which is a divisionof application No. 07/193,964, filed May 13, 1988, now U.S. Pat. No.5,079,334 issued Jan. 7, 1992.

BACKGROUND OF THE INVENTION

The invention relates to the use of polyaniline or derivatives thereoffor absorbing electromagnetic radiation, including microwaves, radarwaves, infrared waves, visible light waves, and ultraviolet waves. Theinvention further relates to the use of the radiation absorbingpolyaniline compositions to modulate another electromagnetic beam. Theinvention also relates to the modification of the electromagneticresponse of polyaniline compositions by chemical or electrochemicalmeans. The invention further relates to electronic and microelectronicdevices based on the chemical and physical properties of polyaniline andits derivatives, and the control of the properties.

When a spectrum of radiant energy is directed into a sample of somesubstances, several things may happen to the energy: (1) it may passthrough the sample with little absorption taking place and therefore,little energy loss. (2) The direction of propagation of the beam may bealtered by reflection, refraction, or diffraction. Scattering of thebeam by particulate suspended matter may also be involved. (3) Theradiant energy may be absorbed entirely or in part. The absorptioninvolves a transfer of energy to the medium, and the absorption processis a specific phenomenon related to characteristic molecular andelectronic structures; the wavelengths of certain components of theradiation may be absorbed while others pass through essentiallyundisturbed, depending on the characteristics of the substance.Components of the radiation are absorbed if its energy matches thatenergy which is required to raise molecular or ionic components of thesample from one energy level to another. Those energy transitions mayinvolve vibrational, rotational, or electronic states. After it has beenabsorbed, that energy may be emitted as fluorescence, utilized toinitiate chemical reactions, or actually dissipated as heat energy.

When molecules interact with radiant energy in the visible andultraviolet region, the absorption consists in displacing an outerelectron in the molecule, although sometimes the energy of the farultraviolet is sufficient to exceed the energy of dissociation ofcertain bonds.

The absorption of radiant energy is a highly specific property of themolecular structure, and the frequency range within which energy can beabsorbed is specifically dependent upon the molecular structure of theabsorbing material. The smaller the energy difference between the groundstate and the excited electronic state, the lower will be the frequencyof absorption (i.e., the longer the wavelength). Chemical compounds withonly single bonds involving sigma-valency electrons exhibit absorptionspectra only below approximately 150 millimicrons. In covalentlysaturated compounds containing heteroatoms, such as nitrogen, oxygen,sulfur, and halogen, unshared p-electrons are present in addition tosigma electrons. Excitation promotes a p-orbital electron into anantibonding sigma orbital, such as occurs in ethers, amines, sulfides,and alkyl halides. In unsaturated compounds absorption results in thedisplacement of pi-electrons. Molecules containing single absorbinggroups, called chromophores, undergo electronic absorption transitionsat characteristic wavelengths, and the intensity of the absorption willbe proportional to the number of that type of chromophore present in themolecule. Marked bathochromic shifts (absorption at longer wavelengths)occur when --OH, --NH₂, and --SH, for example, replace hydrogen inunsaturated groups.

It is desirable for certain applications to have a material whoseradiation absorption characteristics and index of refraction can beeasily and reversibly modulated. Various polymeric materials have beeninvestigated including polyacetylene, polymethylacrylonitrile,pyrazoline, tetracyanoethylene, tetracyanonaphthoquinodimethane,tetracyanoquinodimethane, polydiacetylene, polypyrrole,poly(N-methyl-pyrrole), polyphenylene vinylene, and polythiophene. Someof these polymeric materials are known to exhibit photoresponsiveeffects, but the materials have deficiencies when considered for certainelectromagnetic applications. For example, polyacetylene andpolydiacetylene are nonaromatic, possess unacceptable absorption bandgaps, have limited photoresponse, are air sensitive, generally cannot bederivatized, and are not readily soluble and therefore cannot be easilydeposited as a thin film from solution. In addition, most materialspreviously investigated for electromagnetic radiation absorption are notreadily tunable, i.e., the photoresponses of the materials cannot bereversibly modulated by an external source of energy.

Organic polymers have long been studied for electronic transport and,more recently, for optical properties. The first organic polymersprepared were electrically insulating with conductivities as low as10⁻¹⁴ (ohms cm)⁻¹. The insulating properties are the result of all theelectrons in the polymer being localized in the hybrid-atom molecularorbital bonds, i.e. the saturated carbon framework of the polymer. Theseinsulators, which include polymers such as poly(n-vinylcarbazole), orpolyethylene, have extremely large band gaps with energy as high as 10eV required to excite electrons from the valence to the conduction band.Electrical applications of insulating organic polymers are limited toinsulating or supporting materials where low weight and excellentprocessing and mechanical properties are desirable.

High electrical conductivity has been observed in several conjugatedpolymer or polyene systems. The first and simplest organic polymer toshow high conductivity was "doped" polyacetylene. In the "doped" formits conductivity is in excess of 200 (ohm cm)⁻¹. Although polyacetylenewas first prepared in the late 1950's, it was not until 1977 that thispolyene was modified by combining the carbon chain with iodine and othermolecular acceptors to produce a material with metallic conductivity.

Polyaniline is a family of polymers that has been under intensive studyrecently because the electronic and optical properties of the polymerscan be modified through variations of either the number of protons, thenumber of electrons, or both. The polyaniline polymer can occur inseveral general forms including the so-called reduced form(leucoemeraldine base), possessing the general formula ##STR1## thepartially oxidized so-called emeraldine base form, of the generalformula ##STR2## and the fully oxidized so-called pernigraniline form,of the general formula ##STR3##

In practice, polyaniline generally exits as a mixture of the severalforms with a general formula (I) of ##STR4##

When 0<y<1 the polyaniline polymers are referred to aspoly(paraphenyleneamineimines) in which the oxidation state of thepolymer continuously increases with decreasing value of y. The fullyreduced poly(paraphenyleneamine) is referred to as leucoemeraldine,having the repeating units indicated above corresponding to a value ofy=1. The fully oxidized poly(paraphenyleneimine) is referred to aspernigraniline, of repeat unit shown above corresponds to a value ofy=0. The partly oxidized poly(paraphenyleneimine) with y in the range ofgreater than or equal to 0.35 and less than or equal to 0.65 is termedemeraldine, though the name emeraldine is often focused on y equal to orapproximately 0.5 composition. Thus, the terms "leucoemeraldine","emeraldine" and "pernigraniline" refer to different oxidation states ofpolyaniline. Each oxidation state can exist in the form of its base orin its protonated form (salt) by treatment of the base with an acid.

The use of the terms "protonated" and "partially protonated" hereinincludes, but is not limited to, the addition of hydrogen ions to thepolymer by, for example, a protonic acid, such as mineral and/or organicacids. The use of the terms "protonated" and "partially protonated"herein also includes psueodoprotonation, wherein there is introducedinto the polymer a cation such as, but not limited to, a metal ion, M⁺.For example, "50%" protonation of emeraldine leads formally to acomposition of the formula ##STR5## which may be rewritten as ##STR6##

Formally, the degree of protonation may vary from a ratio of[H+]/[-N=]=0 to a ratio of [H⁺ ]/[-N=]=1. Protonation or partialprotonation at the amine (-NH-) sites may also occur.

The electrical and optical properties of the polyaniline polymers varywith the different oxidation states and the different forms. Forexample, the leucoemeraldine base, emeraldine base and pernigranilinebase forms of the polymer are electrically insulating while theemeraldine salt (protonated) form of the polymer is conductive.Protonation of emeraldine base by aqueous HCl (1M HCl) to produce thecorresponding salt brings about an increase in electrical conductivityof approximately 10¹⁰ ; deprotonation occurs reversibly in aqueous baseor upon exposure to vapor of, for example, ammonia. The emeraldine saltform can also be achieved by electrochemical oxidation if theleucoemeraldine base polymer or electrochemical reduction of thepernigraniline base polymer in the presence of an electrolyte of theappropriate pH. The rate of the electrochemical reversibility is veryrapid; solid polyaniline can be switched between conducting, protonatedand nonconducting states at a rate of approximately 10⁵ Hz forelectrolytes in solution and even faster with solid electrolytes. (E.Paul, et al., J. Phys. Chem. 1985, 89, 1441-1447). The rate ofelectrochemical reversibility is also controlled by the thickness of thefilm, thin films exhibiting a faster rate than thick films. Polyanilinecan then be switched from insulating to conducting form as a function ofprotonation level (controlled by ion insertion) and oxidation state(controlled by electrochemical potential). Thus, in contrast to, forexample, the polypyrrole mentioned above, polyaniline can be turned "on"by either a negative or a positive shift of the electrochemicalpotential, because polyaniline films are essentially insulating atsufficiently negative (approximately 0.00 V vs. SCE) or positive (+0.7 Vvs. SCE) electrochemical potentials. Polyaniline can also then be turned"off" by an opposite shift of the electrochemical potential.

The conductivity of polyaniline is known to span 10 orders of magnitudeand to be sensitive to pH and other chemical parameters. It is wellknown that the resistance of films of both the emeraldine base and 50%protonated emeraldine hydrochloride polymer decrease by a factor ofapproximately 3 to 4 when exposed to water vapor. The resistanceincreases only very slowly on removing the water vapor under dynamicvacuum. The polyaniline polymer exhibits conductivities of approximately1 to 5 Siemens per centimeter (S/cm) when approximately half of itsnitrogen atoms are protonated. Electrically conductive polyanilinesalts, such as fully protonated emeraldine salt [(--C₆ H₄ --NH-- C₆ H₄NH⁺)-C⁻ ]_(x), have high conductivity (10⁻⁴ to 10⁺² S/cm) and highdielectric constants (20 to 200) and have a dielectric loss tangent offrom below 10⁻³ to approximately 10¹. Dielectric loss values areobtained in the prior art by, for example, carbon filled polymers, butthese losses are not as large as those observed for polyaniline.

Polyaniline has been used to coat semiconductor photoelectrodes, toserve as an electrochromatic display material, and to suppress corrosionof iron.

While the preparation of polyaniline polymers and the protonatedderivatives thereof is known in the art, it is novel herein to use thesecompositions for the attenuation of electromagnetic radiation,particularly microwaves, radar waves, infrared waves, visible waves, andultraviolet waves. A need exists for a polymeric material which can bedesigned to absorb microwaves, radar waves, infrared waves, visiblewaves, and ultraviolet waves. In addition, a need exists for a method ofabsorbing the electromagnetic radiation to modulate anotherelectromagnetic beam. A need also exists for a method for themodification of the electromagnetic properties of polyanilinecompositions by chemical or electrochemical means.

SUMMARY OF THE INVENTION

The present invention relates to the use of polyaniline or derivativesthereof for absorbing electromagnetic radiation, including microwaves,radar waves, infrared waves, visible waves, and ultraviolet waves asneeded. The invention further relates to the use of theradiation-absorbing polyaniline compositions to modulate anotherelectromagnetic beam. The invention also relates to the modification ofthe electrical and optical properties of polyaniline compositions bychemical or electrochemical means. The invention further relates toelectronic and microelectronic devices based on the chemical andphysical properties of polyaniline and its derivatives.

While the invention relates to both microwave responses and nonlinearoptical responses of polyaniline and its derivatives, the inventorsbelieve that these phenomena are of different physical origins. Thephotoresponse is believed to be the result of the reorganization ofchemical bonds and to be microscopic. The time frame is believed to beapproximately 10⁻¹³ to 10⁻¹² seconds (a rate of 10¹² to 10¹³ Hz). Theuse of polyaniline compositions to achieve the microwave attenuation ofthe present invention, however, is believed to be due to a localreorganization of the electronic density on the order of 10¹ to 10²Angstroms and on a time frame of approximately 10⁻¹⁰ seconds. Both thephotoresponse and the microwave attenuation phenomena are believed to bedue to the absorption of electromagnetic radiation by the pi electronsystems of the polyaniline polymer and its derivatives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-7 illustrate alternative embodiments of the invention utilizingthe optical properties.

FIGS. 8-11 illustrate waveguides utilizing the microwave absorptionproperties of the invention for absorbing microwaves propagated throughthe waveguide. FIG. 12 illustrates an alternative embodiment in which asurface is coated with a material embodying the present invention forpreventing microwave reflections from the coated material.

FIGS. 13 and 14 illustrate alternative embodiments in which a microwavestrip conductor is coated with material embodying the present invention.

FIGS. 15 and 16 illustrate microwave strip embodiments including anelectrolyte for the controlled variation of the microwave absorptionproperties along the propagation axis of the microwave strip conductors.

FIGS. 17 and 18 illustrate embodiments utilizing thermally responsivefilms which have materials embodying the present invention distributedwithin the film.

FIG. 19 is a graphical plot illustrating the variation of the losstangent as a function of protonation.

DETAILED DESCRIPTION

The dielectric loss of the polyaniline polymeric compositions can,according to the present invention, be controlled by the design of thechemical composition of the polyaniline polymer, the oxidative state ofthe polymer, and the degree of doping, including but not limited to,protonation and pseudoprotonation of the polymer. Thus, by the additionof electron-withdrawing or electron-donating groups to the nitrogenatoms and/or to the C₆ rings of the leucoemeraldine, emeraldine, orpernigraniline polyaniline compositions, the dielectric loss tangent canbe varied. For example, addition of a methyl group to each C₆ ring toform poly(orthotoluidine) leads to a dielectric loss tangent that can bevaried from 10⁻² to 10⁰. By the present invention dielectric losstangents can be varied from 10⁻² to approximately 20 by varying the formof the polyaniline, the degree, site and type of substituents. In theprior art, carbon filled silicone rubber or carbon filled epoxy paintsor carbon bonded to fabric produce non-magnetic dielectric losses atmicrowave frequencies. A preferred embodiment of the present inventionfor attaining maximum dielectric loss is the emeraldine salt, wherein yis in the range of from approximately 0.4 to 0.6 and the protonation isapproximately one proton per imine nitrogen, i.e., [H⁺ ]/[-N=] is equalto approximately one.

The addition of electron-withdrawing or electron-donating groups to thepolyaniline composition can facilitate the design of a polymericmaterial with desired absorption and transmission bands. Knownelectron-donating groups to be substituted onto the C₆ ring andoperative in the present invention can include, but are not limited to,--OCH₃, --CH₃, halogens (electron-donating by way of a resonanceeffect), --NR₂, --NHCOR, --OH, --O⁻⁻, --SR, --OR, and --OCOR. Thesegroups or atoms possess one or more unshared electron pairs on the atomadjacent to the ring. Known electron-withdrawing groups can includehalogens (electron-withdrawing by way of an inductive effect), --NO₂,--COOH, --COOR, --COR, --CHO, and --CN. Thus, the addition ofelectron-donating groups to the rings of polyaniline augments the chargedelocalization. The added opportunities for resonance stabilization ofthe pi to pi* excited state provided by electron-donating groups causesa marked lowering in the requirement for excitation energy, and thus adecreased frequency (longer wavelength) of absorption. Conversely, theaddition of electron-withdrawing groups diminishes the opportunities forresonance stabilization, causing an increase in the requirement forexcitation energy, and thus an increased frequency (shorter wavelength)of absorption. Thus, for example, protonation of --NH₂ changes it to--NH₃ +; this group no longer has an unshared pair of electrons toparticipate in charge delocalization. Alteration of --OH to the ion,--O--, provides further opportunity for participation of unsharedelectrons on oxygen in charge delocalization. Thus, the change of H toNH₂ is bathochromic; NH₂ to NH₃ + is hypschromic; OH to O⁻ isbathochromic; and both of the changes, OH to OCOCH₃ and NH to NHCOCH₃(acetylation), are hypsochromic.

The electron-withdrawing or electron-donating group can be present onthe C₆ rings or the nitrogen atoms of the polyaniline composition at anydesired percentage of the available sites. The electron-withdrawing orelectrondonating groups are added to the C₆ ring sites or the nitrogenatom sites by chemical techniques known to those skilled in syntheticorganic chemistry.

In this manner, a polyaniline composition is prepared which whenproduced in a flexible sheet form or which is coated onto a flexiblesubstrate can be used to absorb electromagnetic radiation. Thus, a meansof rendering an object undetectable to electromagnetic radiation such asradar is produced by the present invention by draping over the objectthe flexible polyaniline film or the coated flexible substrate, such asa cloth fabric or fishnet. Furthermore, by coating electromagneticradiation-absorbing polyaniline compositions onto fibers, and thenproducing woven or non-woven fabrics from the coated fibers, cloth orclothing which is radiation absorbing could be produced, according tothe present invention. In another embodiment, fibers of polyanilineitself or a derivative thereof, or fibers of polyaniline copolymerizedwith another polymer can be drawn or extruded and subsequently woveninto electromagnetic radiation absorbing fabric, garments, coverings,and the like. In this manner radar absorbing clothing can be produced.

A further advantage of the present invention is that the polyanilinecompositions and derivatives thereof have, or can be designed to have,desired processability in terms of, for example, viscosity, flexuralstrengths, solubility, adhesion to substrates, crosslinking, meltingpoint, weight, adaptability to filler loading and the like. This isachieved by varying as desired the degree of protonation, the state ofoxidation, and the type and degree of substituents on the polymer.Certain substituents may be preferred for the facilitation of desiredprocessing parameters, such as increasing or decreasing solubility,altering extrusion parameters (rheology), achieving a specificviscosity, and the like. Derivatization is also useful for achievingcompatibility with a copolymer, facilitating the tunability of thepolyaniline composition for non-linear optics applications, and forspecific wavelength absorption, such as microwave attenuation or aparticular photoresponse.

The polyaniline compositions useful in the present invention can becoated by a variety of techniques onto substrates of choice. Thepolyaniline polymers can be applied to substrates according to thepresent invention by spray coating, dip coating, spin casting, transferroll coating, brush-on coating, and the like. The polyaniline polymerscan also be electrochemically deposited onto conductive substrates byknown electrochemical deposition techniques.

According to the present invention, polyaniline can also be entrainedwithin a matrix of, or copolymerized with, another polymer material tothereby produce a blend or a composite. Thus, polyaniline could bedispersed in, for example, polyethylene, polyimide, cellulose nitrate,and the like, and also can be coated onto fibrous materials. Inaddition, derivatization of the polyaniline compositions can enhancecompatibility and processability of the polymer with other polymers.

In addition, the polyaniline compositions can be cast as thin films froma solvent solution, and the solvent evaporated to produce free standingfilms. The polyaniline films can be stacked as a composite with otherpolyaniline films, with films of polyaniline copolymerized with anotherpolymer, or with non-polyaniline polymers and/or copolymers. Dependingon the desired type and degree of substitution of the polyaniline withvarious crosslinkable functional moieties, the films produced can becured in deeper sections, that is, thicker films or articles can also beproduced by known polymer preparation techniques. Such thickerpolyaniline materials will have some utility in certain non-linearoptics applications, but will be even more preferred in certainradiation absorption applications, such as microwave attenuation.

Polyaniline will absorb electromagnetic radiation in the visiblespectrum, in the infrared range, and in the ultraviolet range. Thus, thepresent invention further relates to a method of absorbing infrared,visible, or ultraviolet waves comprising exposing the polyaniline toinfrared, visible, or ultraviolet waves, whereby the infrared, visible,or ultraviolet waves are absorbed by the polyaniline. The presentinvention also relates to a method for absorbing microwave radiationcomprising exposing polyaniline to microwave radiation, whereby themicrowave radiation is absorbed by the polyaniline.

Because polyaniline compositions are shown by the present invention toabsorb electromagnetic radiation, another object of the invention iselectromagnetic shielding. A thin film of polyaniline within, forexample, the walls of television sets, computers, electronic machinery,and places for the storage of electronic data, such as computersemiconductor memories, will effectively absorb continuous andintermittent electromagnetic radiation from wires, coils, cathode tubes,etc. Protection against unwanted or unknown electronic surveillance ofrooms can be achieved by the application of polyaniline to the walls,floor, and ceiling. Similarly, electrical wires can be shielded by theincorporation of a layer of polyaniline material into the plasticinsulator coating on the wires with the advantage of grounding andstatic free property.

In addition, polyaniline can be used to make a remote thermal switch byexposing the polyaniline composition to microwave radiation. Thepolyaniline composition absorbs the radiation, which heats up thepolyaniline, which in turn, can trigger a thermocouple placed in contactwith the polyaniline composition. Upon removal of the source ofmicrowaves, the polyaniline composition will cool and cause thethermocouple to switch back. By this manner a thermal switch isproduced.

Thus, the present invention relates to a composition for absorbingelectromagnetic radiation, wherein said electromagnetic radiationposseses a wavelength generally in the range of from about 1000Angstroms to about 50 meters, wherein said composition comprises apolyaniline composition of the formula I, above, or a protonated saltthereof, where y is in the range of approximately 0.2 to 0.8, and thedegree of protonation, i.e., ×=[H⁺ ]/[-N=], varies from ×=0 through ×=1.

The instant invention further relates to a method of applying heat to asubstrate, said method comprising the steps of:

(a) applying to a substrate a microwave radiation-absorbing polyanilinecomposition, or a partially protonated salt thereof;

(b) exposing the microwave radiation-absorbing polyaniline composition,for example, a partially protonated salt thereof, to microwaveradiation, whereby the microwave radiation-absorbing polyanilinecomposition, or the partially protonated salt thereof, absorbs themicrowave radiation, resulting in the generation of thermal energywithin the polyaniline composition. This heat can be localized,transferred from the polyaniline composition or the salt to a substrateand utilized to accomplish desired results, such as, but not limited to,joining of materials. Thus, two materials which have been placed incontact or close proximity with each other and in contact with apolyaniline composition can be adhered to each other upon the exposureof the polyaniline composition to sufficient microwave radiation to heatand thus melt or at least soften at least one of the materials to enablefusing. The frequency, duration and/or intensity of the microwaveradiation necessary to achieve the desired adhesion of the two materialswill vary depending on the nature of the materials to be adhered and onthe degree and type of protonation and/or substitution, if any, on thepolyaniline. The preferred frequency of the microwave radiation to beabsorbed by the polyaniline compositions to thereby induce localizedheating is from about 10⁹ Hz to about 10¹¹ Hz. The polyanilinecomposition can be applied to one or both of the materials in anypattern, such as a grid pattern, stripes, spots, or the like as desired.The polyaniline can be applied via solution coating, adhesion of films,vapor deposition, extrusion of gels containing polyaniline, and otherknown application techniques.

In a preferred embodiment of the present invention directed toward theadhering of two or more materials by the absorption of microwaveradiation by polyaniline, at least one of the materials to be adhered isa plastic. In another embodiment of the present invention one of thematerials to be adhered is a silicate-containing material, such as, forexample quartz or glass. In this manner, a plastic can be adhered to aglass fiber, such as an optical fiber, by means of exposure of thepolyaniline to microwave radiation.

According to the present invention, polyaniline compositions can also beutilized to absorb radar waves possessing a wavelength in the generalrange of from about 0.01 cm to about 100 cm. The absorption of radarwaves by the polyaniline composition would assist in rendering objectscoated with the polyaniline composition relatively invisible to radardetection. Therefore, the instant invention further relates to a methodfor absorbing radar waves comprising exposing a polyaniline compositionor a partially protonated salt thereof to radar waves whereby thepolyaniline composition or the salt thereof absorbs at least some of theradar waves. The invention further relates to a method for reducing thedetectability by radar of an object comprising applying to the object apolyaniline composition or a partially protonated salt thereof in anamount sufficient to absorb at least some, and preferably all, radarradiation to which the object may be exposed.

In a preferred embodiment of the method for reducing the detectabilityby radar of an object it is desirable to coat the object in such a wayas to produce a gradient of absorption to minimize reflectance. Such agradient of polyaniline material can be achieved by varying the degreeof protonation of the polymer or the degree of substitution on eitherthe C₆ ring or the nitrogen atoms or both with a chemical substituentsuch that an incoming radar beam first encounters a polyanilinecomposition with little or no protonation, i.e., a material with limitedabsorption of radiation. As the beam further advances along the gradientof polyaniline material covering the object, the beam encounterspolyaniline polymer with continually increasing degrees of protonation,and hence increasing degrees of electromagnetic absorption. In thismanner, little or no reflection of the beam is produced and the objectis not detectable by a radar wave reflection.

The present invention further relates to a method of electrochemicalswitching of the polymeric state of a polyaniline composition. Bycontacting the polyaniline composition with an electrolyte,electrochemical switching of the polymeric state can be significantlyaccelerated, being accomplished on a time scale of approximately 10⁻⁵seconds. By contacting the polyaniline composition with a solidelectrolyte, electrochemical switching of the polymeric state can beeven further accelerated, being accomplished on a time scale of lessthan approximately 10⁻⁷ seconds. For electromagnetic radiationabsorption, such as the absorption of microwave radiation,electrochemical switching of the polymeric state can turn the polymericmaterial from radiation transparent to radiation absorbing, or viceversa, depending on the nature and direction of the electrochemicalswitching. For non-linear optics, electrochemical switching can changethe important absorption and/or transmission bands for the probe andmodulator beams, such as, for example, in switching from the emeraldinebase form to the emeraldine salt form of polyaniline. The range of theabsorption bands for the base and the salt can be shiftedbathochromically (i.e., shifted to longer wavelengths) orhypsochromically (i.e., shifted to shorter wavelengths) as may bedesired according to the characteristics of the available probe beam,the available modulator beam, or the available detector or sensor, orany combination thereof.

Polyaniline compositions can also be used according to the presentinvention as a photoactive switch by manipulation of the index ofrefraction of the polyaniline compositions. Because of the extremelyrapid photoresponse of the polyaniline polymer, it is therefore usefulaccording to the present invention in nonlinear optical devices. Thetime dependence of the photo bleaching of the polymer is on the order ofpicoseconds. For example, the application of a laser beam of wavelength6250 Angstroms (2.0 eV) to polyaniline polymer produces significantphotoinduced bleaching (i.e., increased transmission) in broad energybands of 8,265 Angstroms to 4,590 Angstroms (approximately 1.5 eV to 2.7eV) and again at 3,760 Angstroms to 2,880 Angstroms (approximately 3.3eV to 4.3 eV). Simultaneously laser beam photoinduced absorption (i.e.,decreased transmission) for polyaniline occurs at 24,800 Angstroms to8,265 Angstroms (approximately 0.5 eV to 1.5 eV) and from 4,590Angstroms to 3,760 Angstroms (2.7 eV to 3.3 eV). Photoinduced absorptionand bleaching occur in polyaniline compositions in less than 10⁻¹²seconds. These photoinduced changes in absorption correspond to changesin the index of refraction at these wavelengths. These changes inoptical constants have broad application in nonlinear optical signalprocessing and optical communications, which according to the presentinvention, are useful as means to switch, modulate, multiplex, focus,and provide optical bistability for commercial systems.

Polyaniline is therefore useful in nonlinear optical signal processingaccording to the present invention. For example, a thin film coating ofpolyaniline can be applied to a phototransmissive substrate. In oneembodiment of the present invention, a probe beam of light of a givenwavelength is then propagated through the noncoated side of thesubstrate onto the coating at the critical angle to the polyaniline suchthat the probe beam is wave-guided in the phototransmissive substrate.To activate the desired switching property of the polyaniline coating, apump beam of light, also called a modulator beam, of a differentwavelength or some wavelength is applied to the coating through thecoated or noncoated side of the substrate at a second angle such thatthe index of refraction of the polyaniline composition is changed by theabsorption by the polyaniline of the electromagnetic radiation of themodulator beam. The wavelength of the modulator beam can vary widely,but is preferably within the range of from about 12,100 Angstroms (1.5eV) to about 21,775 Angstroms (2.7 eV). The change in the refractiveindex of the polyaniline composition coating alters the transmissiveproperty of the polyaniline and allows the probe beam to be refracted orotherwise modified by the polyaniline coating. This refraction or othermodification of the probe beam can, for example, be used to trigger aphotocell, initiate or terminate an optical signal, encode informationon the probe beam, or the like. By these means is produced a low cost,stable means of optical signal processing.

In an alternative embodiment, the beam to be modulated is refracted bythe phototransmissive substrate and reflected off the polyanilinecoating on the backside of the substrate such that the beam is thenreflected repeatedly between the front side of the substrate and thepolyaniline coated back side of the substrate. This reflection continueswithin the phototransmissive substrate until the modulating beam iscaused to impinge on the polyaniline coating, whereby the index ofrefraction of the polyaniline coating is altered by the absorption ofthe electromagnetic radiation of the modulator beam, altering thepropagation of the probe beam. In this manner the polyaniline coatinghas acted as a switch which is reversibly controlled by the presence ofthe pump or modulating beam to increase or decrease the modulation (bothintensity and direction) of the probe beam. Because of the very rapidphotoresponse rate of the polyaniline polymer, the refractive index canbe altered at gigahertz to terrahertz rates, thereby providing a methodfor the rapid modulation of optical data signals.

In yet another preferred embodiment, the beam to be modulated is causedto impinge upon a thin coating of polyaniline which is on aphototransmissive substrate. A portion of the beam is reflected, theremainder refracted, transmitted, and partly absorbed. Application of amodulator beam at a second angle changes the index of refraction of thepolyaniline thereby altering the direction and the percentage of theprobe beam transmitted and reflected. The preferred embodiment has theprobe beam incident on the polyaniline at the critical angle and themodulator beam preferably of wavelength between 12,100 Angstroms (1.5eV) and 21,800 Angstroms (2.7 eV).

Thus, the present invention further relates to a method of changing therefractive index of polyaniline comprising the steps:

(a) applying polyaniline to a phototransmissive substrate;

(b) applying a first beam of light of wavelength x at the critical angley to the polyaniline surface; and

(c) applying a second beam of light of wavelength z to the polyanilinesurface, whereby the second beam is absorbed by the polyaniline changingthe index of refraction of the polyaniline, whereby the transmission ofthe first beam through the phototransmissive substrate is altered. Thepreferred wavelength x of the first or probe beam of light is dependenton the form of polyaniline utilized. For emeraldine base polymer, thepreferred wavelength x of the first or probe beam of light is in one ormore of the ranges of approximately 0.6 eV to 4.2 eV; 0.8 to 1.1 eV; 1.6to 2.4 eV; 2.8 to 3.2 eV: and 3.4 to 4.3 eV. The preferred wavelengthswill vary depending on the degree of protonation of the polyanilinepolymer and the nature of the substituents, if any, on the polymer. Forthe emeraldine base polymer, the preferred wavelength z of the second ormodulating beam of light is in the range of approximately 1.7 eV to 2.7eV. The preferred wavelength of the second or modulating beam isdetermined by the oxidation state, protonation level, and substituentsof the polymer. For the leucoemeraldine polymer the preferredwavelengths of the probe beam are in the range of 24,800 Angstroms to8,265 Angstroms (0.5 to 1.5 eV) and 4,590 Angstroms and 3,760 Angstroms,with greater preferred modulator beam wavelength of 3,760 Angstroms to2,880 Angstroms. For pernigraniline, the preferred probe and modulatorwavelength are similar to emeraldine.

The photoswitching phenomenon can, according to the present invention,also be used to couple a light signal from one optical fiber to anotheroptical fiber. The two optical fibers are positioned in close contactwith each other and with a thin film of polyaniline composition betweenthem. The polyaniline composition is then exposed to a modulating beam.The modulating beam changes the index of refraction of the polyanilinesuch that "crosstalk" between the two optical fibers is obtained. Thisallows the optical signal within either of the optical fibers to becoupled to the other fiber as desired, but without permanent physicalalteration of either fiber. In addition, the coupling can be turned onand off as desired by the manipulation of the index of refraction and,because of the very rapid photoresponse rate of the polyaniline polymer,the refractive index can be altered and coupling achieved at gigahertzto terrahertz rates.

In yet another embodiment of the present invention the polyanilinecomposition can itself be utilized simultaneously as thephototransmissive material and a photoswitch without a phototransmissivesubstrate. Thus, a free standing polyaniline polymer can be exposed to afirst beam of light which will be transmitted through the polyanilinewith some attenuation. When the polymer is exposed to a second ormodulator beam the refractive index and absorption coefficient of thepolyaniline polymer are altered, changing the intensity and angle ofrefraction of the beam transmitted through the polymer.

Another embodiment of the present invention is the use of polyanilinecompositions as a masking material over ultraviolet-curable polymers inthe fabrication of positive resist and negative resist microelectronicdevices and circuits. In the fabrication of certain positive resist andnegative resist microelectronic devices and circuits, radiation curablepolymers are deposited on conductive or semiconductive surfaces, such assilicon or doped silicon. A circuit pattern is then applied by means ofphotolithographic techniques and covered by ultraviolet-curable polymersin certain desired patterns. Ultraviolet radiation is then applied tothe polymers to cure certain portions, after which the uncured portionsare removed by solvent rinsing, for example. In this manner, patterns ofcured polymer are provided on the conductive or semiconductive surfaces.By the present invention, polyaniline can be applied to the curablepolymer in a predetermined pattern such that the polymer beneath thepolyaniline pattern is desired to remain uncured upon exposure of thecoated device or circuit to radiation. When the polymer is exposed tothe radiation, the polyaniline would absorb the ultraviolet radiation tothereby mask the polymer and prevent the cure in certain locations ofthe curable polymer beneath.

Thus, the present invention relates to a method for masking a radiationcurable polymer applied to an electronic circuit or device, said methodcomprising the steps of (a) applying a radiation-curable polymer orprepolymer to an electronic device or circuit; (b) applying to theradiationcurable polymer or prepolymer a polyaniline composition; (c)exposing the device or circuit with the curable polymer or prepolymerand the polyaniline composition to radiation sufficient to cure thecurable polymer or prepolymer and whereby the polyaniline compositionabsorbs some of the ultraviolet radiation; and (d) removing thepolyaniline and any uncured curable polymer or prepolymer. In apreferred embodiment of the invention, the curable polymer or prepolymerand the polyaniline are independently deposited onto the surface of theelectronic device or circuit by means of a solvent solution of eachmaterial, followed by the evaporation of the solvent. By "cure" hereinis meant sufficient correction and/or crosslinking reactions have takenplace to render the material a solid not easily removed by solvent.

We have developed a series of devices which utilize the features,characteristics and properties of the polyaniline compounds which aredescribed above. First is a series of optical devices and second aseries of microwave devices.

The optical devices are useful in a range of the electromagneticspectrum at or near what is commonly referred to as light. These devicesutilize the fact that the index of refraction of the polyanilinecompounds may be controlled by varying the intensity and wavelength oflight radiated upon the polyaniline compound. Thus, a pumping ormodulating light at one or a broad band of frequencies may be used tomodulate the index of refraction of the polyaniline compound and therebymodulate light at another frequency. For example this can be used forthe coupling of the modulated light from one light transmissive mediumto another or modulating its angle of departure from an interfacebetween two light transmitting media. The spectral response of thisphoto effect is substantially changed as the compounds are more fullyprotonated with the largest response in the unprotonated material. Thus,the unprotonated polyaniline bases are preferred for some applications.

We have devised devices which can operate as various types of lightvalves, as a phase velocity modulator and for con-trolling the angle ofemission of a light beam. The light valves may be light switches turningthe modulated beam on or off or variable valves which permit theintensity of the modulated light beam to be varied continuously over arange by varying the intensity of the modulating light beam. In a valvethe modulating light beam pumps the electrons into higher energy bandscausing the critical angle for a light beam incident upon an interfaceto be increased for some frequency bands and decreased for other bandsas the intensity of the pump or modulating light increases.

FIG. 1 illustrates one such device. It has a substrate 110 supporting apolyaniline mass 112 in the form of a film bonded to the substrate 110.The interface between the polyaniline film 112 and the air will have acritical angle of, for example, C1 when the modulating light 114 doesnot pump the polyaniline film 112 and a critical angle C2 when the pumplight 114 is intense. The angles are greatly exaggerated forillustration. In this example when the modulating light 114 does notpump the polyaniline film 112, a light beam 116 which is incident uponthe polyaniline film at a angle greater than the critical angle C1 willnot be substantially transmitted into the film 112 but instead will bereflected along a path 118. However, when the modulating beam 114 isturned on for pumping, the critical angle increases to angle C2 thuspermitting the coupling of light from the beam 116 into the film 112.

We have found that the critical angle is increased by an increase in theintensity of the modulating beam for some frequency ranges of incidentmodulated light and is decreased for others. Thus, the illustration ofFIG. 10 continues to be accurate for all events. However, for somefrequencies of incident, modulated beam, the critical angle when thepolyaniline is not pumped is the greater angle and then decreases to C1as pumping energy is increased.

The result is that the incident light beam is always substantially equalto the critical angle being either slightly greater or lesser than theprecise critical angle depending upon the incident beam frequency. Thus,the structure can be used so that increasing the intensity of thepumping, modulating beam 114 will turn off the coupling of light fromone transmission medium to another for some optical bands while turningon the coupling for other bands.

FIG. 2 illustrates a structure utilizing the same principles asillustrated in FIG. 10 but for coupling the light from the polyanilinefilm 120 into another light transmissive medium 122. However, in thestructure of FIG. 11, the interface at which the critical angle isimportant is the interface between the polyaniline film 120 and thelight conductor 122. Thus, in the embodiment of FIG. 11 the incidentlight beam 124 must be incident at an angle such that after it entersthe polyaniline film and is refracted along a different path it willapproach the interface between the polyaniline film 120 and thetransmissive substrate 122 at substantially the critical angle, beinggreater or lesser than the critical angle by a smaller amount inaccordance with the principles described in connection with FIG. 1. Thecoupling of light from the polyaniline layer of 120 to the other layeris thus controlled by the modulating light 126. From the abovedescription it is apparent that a beam in the opposite direction mayalso be similarly controlled.

FIG. 3 illustrates another embodiment similar to the embodiment of FIGS.1 and 2. In FIG. 3, however, an optical fiber light conductor 130 has apolyaniline layer 132 upon at least a portion of its outer longitudinalsurface. In this manner a pumping light 134 can control the coupling ofan incident light beam 136 into the optical fiber 130.

FIG. 4 illustrates an optical fiber 140 having an endface 142 which islapped at substantially the critical angle for the interface between theoptical fiber 140 and a polyaniline layer 144 coated on the lappedendface. An incident light beam 146 may be directed upon the polyanilinefilm 144 parallel to the longitudinal optical axis of the optical fiber140. The device operates on the same principles described in connectionwith FIGS. 1 and 2 except the light beam when coupled into the opticalfiber 140 enters along the longitudinal optical axis.

FIGS. 5 and 6 illustrate yet another device in which a pair of opticalfibers 150 and 152 are controllably coupled together by an interposedpolyaniline mass 154 joining the two fibers. Together these form threelight conducting media. An input light beam 156 propagating along thefiber 150 enters the region along which the polyaniline 154 isdistributed. The coupling of light into the polyaniline 154 and into thesecond optical fiber 152 is controlled by the pumping beam 158 so thatsome of the light from input light 156 is coupled into optical fiber 152to provide as output light 159.

FIG. 7 illustrates a Mach Zehnder interferometer made up of lightconductors in which input light 160 arrives in optical fiber 162 and isdivided into two paths 164 and 166. These two paths are recombined atoptical fiber 168. In accordance with the principles of the prior artMach Zehnder interferometer, if the two beams arrive in phase in theoutput fiber 168 they constructively interfere and the light beam andany associated signal continues along the path. However, if the beamsdestructively interfere, the light beam is destroyed. By variablycontrolling or altering the phase velocity through the branch 166, therelative phases of the two light signals recombining at output opticalfiber 168 may be controllably varied between constructive anddestructive interference.

While the Mach Zehnder interferometer is old and known, we have found anew manner of controlling the phase velocity in the branch 166. Thebranch 166 is coated with a layer of the photo responsive polyanilinefilm 170. A variable intensity pumping light 172 for modulating theindex of refraction of that layer is used to control the phase velocityof the light through the branch 166. Thus, varying the intensity of themodulating or pumping light 172 changes the phase velocity in the branch166 and therefore changes the phase relationship between the twoarriving signals in the output fiber 168.

The microwave devices of the present invention arise because we havediscovered that a highly protonated emeraldine salt polymer has a highdielectric loss which we attribute to its combination of a modestconductivity and a high dielectric constant. The loss tangent, aquantitative indication of the energy loss in the polymer, is a functionof the protonation level of the polymer and increase as the protonationlevel increases reaching a maximum at complete 50% protonation. Veryimportantly, the protonation level may be controlled as described abovein the synthesizing of the material and also may be varied by changingthe potential by means of an electrochemical cell. This permits thepolymers to be made with a variety of selected loss tangents and furtherpermits the loss tangents of the polymers to be variably controlled in avariety of useful devices. For example the microwave properties may beturned on and off or varied over a range.

In FIGS. 8 and 9 a layer 180 of protonated polyaniline is bonded to theinterior walls of a wave guide 182. As the microwave propagates from theinput end 183 to the output end 184, it is attenuated in the polyanilinelayer. This layer may be formed with a continuously changing protonationlevel along the propagation axis of the waveguide to provide a selectedprotonation gradient and therefore loss gradient between the input end183 and output end 184. This gradient can be contoured to minimizereflections where the propagating microwave encounters the transitionfrom an absence of polyaniline layer to the presence of the layer.Additionally, the layer may be geometrically formed to gradually taperto a greater thickness as the microwave propagates from the input end183 to the outlet end 184.

FIG. 10 illustrates a similar wave guide 186 having a mass ofpolyaniline 188 formed so that it has a geometrical configurationproviding an increase in thickness, that is an increase in its crosssectional area in planes perpendicular to the axis of propagation.

FIG. 11 illustrates another embodiment of the invention in which thepolyaniline mass is positioned within the interior of a waveguide 190and also extends between its walls. In the interior cf the embodiment ofFIG. 11 there is positioned a plurality of laminated layers 192 ofpolyaniline each layer having a different protonation level. Thisstructure is particularly suitable for terminating the end 194 of awaveguide 190 in a manner to prevent reflections. Other circuitry may beused to direct unwanted microwave energy, for example, into theillustrated wave guide where it can be effectively attenuated. In orderto minimize reflections, as described above, the layers initiallyencountered by the incoming microwave are the least protonated so theyare the less absorptive. The layers become increasingly more protonatedand therefore more absorptive as they are positioned closer to the end194 of the wave guide 190. Preferably, the average gradient of thevariation in protonation and therefore in the variation in absorption isapproximately a linear function of distance along the propagation axis.

Referring to FIG. 12, if it is desired to prevent reflections ofmicrowave energy from a metallic or other reflective surface 196, thesurface may be coated with a polyaniline layer 198 which is providedwith a protonation gradient which increases from near 0% at the exposedouter surface 200 to a much greater level, 50% for example, at theinterface 202 between the reflection surface 196 and the polyanilinelayer 198.

Similarly, as illustrated in FIGS. 13, 14 and 15 the polyanilineabsorptive layer may be bonded to the exterior surface of a microwavestrip conductor such as conductor 206 in FIG. 13. This provides aconvenient means for introducing attenuation onto a microwave stripconductor used in miniature or integrated circuits while minimizingreflections from it. For example, the polyaniline layer 208 may besynthesized as described above so that it has a variation in itsprotonation or pseudo-protonation as a function of its position ordistance along the axis of propagation of the strip conductor. Theprotonation varies from a minimum protonation level at its opposite ends210 and 212 increasing toward the central region to a maximumprotonation at the central region to 214.

FIG. 14 illustrates use of the polyaniline as a terminating absorber onthe end of a branch of the microwave strip conductor 220. Thepolyaniline layer 222 is formed with a protonation gradient extendingfrom a minimum protonation at its input end 224 to a maximum protonationat its terminating end 226.

The gradual variation in the microwave loss tangent so that absorptionincreases gradually from the input end to the opposite end along thepropagation axis may alternatively be accomplished or may besupplemented by increasing the thickness of the polyaniline layer toalso increase energy absorption.

One major advantage of polyaniline materials used in the presentinvention is that their protonation and therefore their absorption orloss tangent may be controlled by an electrical potential. This featuremay be utilized in many various embodiments of the invention but isillustrated in FIG. 15.

FIG. 15 illustrates a microwave strip conductor 230 upon which apolyaniline layer 232 is positioned of the type illustrated in FIG. 13.The strip may, if desired, have a protonation or a thickness gradient.This layer, because it is also conductive, may also serve as oneelectrode of an electrochemical cell. It is connected to a variablepotential 234. The other terminal of the variable potential 234 isconnected to the other electrode 236 of this electrochemical cell. Asolid or liquid electrolyte 238 is positioned between the electrodes 232and 236. Thus, the application of the variable potential 234 permits thepotential of the polyaniline 232 to be varied, controlling or varyingits protonation as a function of the potential and thereby varying itsabsorption or loss tangent. In this manner, the effect of thepolyaniline layer 232 may be switched on and off by switching thepolyaniline between full protonated and unprotonated states and may bevaried to intermediate levels of protonation.

FIG. 16 illustrates a structure for controlling and varying theprotonation gradient along a microwave strip conductor 240 by formingthe polyaniline layer into a series of discrete segments 241 and 242along the axis of propagation of the microwaves. Each individualpolyaniline segment forms an electrode of a separate electrochemicalcell. Each of these cells has a second electrode 244 and 246, forexample, and an interposed electrolyte like the electrolyte 238 in FIG.25 but separately associated with each individual electrochemical cell.A separate potential is applied for controllably varying the potentialof each discrete segment of polyaniline mass along the propagation axisso that its loss tangent may be independently varied. Differentpotentials may be applied to each of these discrete cells tocontrollably contour the protonation gradient which is desired forparticular circumstances.

FIGS. 17 and 18 illustrate embodiments in which a thermally responsivefilm has microwave radiation absorbing protonated polyaniline saltpolymer distributed in the thermally responsive film. The termdistributed is intended to include the various chemical techniques fordistributing the active materials in a thermally responsive filmmaterial, this includes dispersing and copolymerization. Thisdistribution of the microwave radiation absorbing polymer in thethermally responsive film permits the film to be activated by microwaveenergy rather than by radiation with infrared energy. This isparticularly useful in electrically insulative environments in which theenergy may be coupled specifically into the thermally responsive filmwithout undu heating of surrounding structure and from a remote source.

For example, if the principal carrier film is a thermally deformablefilm such as thermo-plastic film which softens and flows more easilywhen heated, it may be used to form a barrier or closure which can bedestroyed from a remote position by irradiation with microwave energy.For example, FIG. 17 illustrates a conduit 250 having a thermoplasticfilm 252 with microwave radiation absorbing protonated polyaniline saltpolymer distributed within it to form a barrier between the conduit 250and a conduit 254. This sealing film will block passage of fluids, forexample, past the barrier formed by the film 252 until the film isradiated by microwave energy 256 causing the barrier to be heated,softened and eventually separate to open the passage.

The microwave radiation absorbing protonated polyaniline salt polymermay also be distributed in or copolymerized with a conventionalthermally activated shrink wrap film. For example, FIG. 18 illustrates aconduit 260 having a defect or crack 262. The film 264 is looselywrapped around the conduit 260 and the film 264 is then irradiated withmicrowave energy 266 causing the polyaniline polymer to absorb themicrowave energy convert it to heat thereby heating the film andactivating its shrink properties.

The microwave absorptive polyaniline film can be advantageously used forshielding objects as mentioned above and is particularly useful forshielding a plurality of electrical conductors to form a shielded cable.The polyaniline shield not only prevents electromagnetic energy fromentering the cable and thereby coupling noise into the conductors andprevents electromagnetic energy from exiting the cable but additionallybecause the polyaniline is also conductive, the outer conductingpolyaniline shield may also be used to ground an electronic device towhich it is connected. Thus, because the polyaniline is also conductive,it may not only absorb rather than merely reflect the microwave energybut it may also conduct current to maintain an electrical apparatus at aground potential.

In order to measure the variation in loss of the polyaniline polymer asa function of protonation, we measured the loss tangent for fivedifferent samples spaced across the range between the emeraldine base atno protonation and the emeraldine salt at full 50% protonation. FIG. 19is a plot of the results of the data illustrating the loss tangent inthe vertical axis as a function of percent of protonation on thehorizontal axis. For each sample there is illustrated a data point aswell as error bars indicating the estimated accuracy with which themeasurements were made. This graph illustrates the increase in losswithin the polymer as protonation is increased.

While the invention has been disclosed in this patent application byreference to the details of preferred embodiments and examples of theinvention, it is to be understood that this disclosure is intended in aillustrative rather than in a limiting sense, as it is contemplated thatmodifications will readily occur to those skilled in the art, within thespirit of the invention and the scope of the claims which follow.

That which is claimed is:
 1. A method of absorbing electromagneticradiation comprising supporting a polyaniline composition in a positionto expose the composition to a source of electromagnetic radiation,whereby the electromagnetic radiation is absorbed by the polyanilinecomposition.
 2. A method of absorbing electromagnetic radiation asclaimed in claim 1 comprising supporting a polyaniline composition in aposition to expose the composition to a source of infrared radiation,whereby the infrared radiation is absorbed by the polyanilinecomposition.
 3. A method of absorbing electromagnetic radiation asclaimed in claim 1 comprising supporting a polyaniline composition in aposition to expose the composition to a source of visible lightradiation, whereby the visible light radiation is absorbed by thepolyaniline composition.
 4. A method of absorbing electromagneticradiation as claimed in claim 1 comprising supporting a polyanilinecomposition in a position to expose the composition to a source ofultraviolet radiation, whereby the ultraviolet radiation is absorbed bythe polyaniline composition.
 5. A method of absorbing electromagneticradiation as claimed in claim 1 comprising supporting a polyanilinecomposition in a position to expose the composition to a source ofmicrowave radiation, whereby the microwave radiation is absorbed by thepolyaniline composition.
 6. A method for absorbing electromagnetic wavescomprising supporting a polyaniline polymeric composition or a partiallyprotonated salt thereof in a position to expose the composition or saltto a source of radar waves whereby the polyaniline composition or thesalt thereof absorbs the radar-waves.
 7. A method for reducing thedetectability by radar of an object by increasing the absorption ofradar waves and thereby decrease the reflection of the radar wavescomprising applying to the object a polyaniline composition or apartially protonated salt thereof in an amount sufficient to absorb atleast some radar radiation to which the object may be exposed, therebyreducing the detectability by radar of the object.